1. Field of the Invention
This invention relates to a dialer circuit, and more particularly to the improvement to a dialer circuit including a dialer memory for sending out a dialer output signal to a telephone circuit line based on an input signal and also for storing therein at least part of the dialer output signal. In the dialer circuit, a supply voltage taken from a telephone circuit line is supplied to a dialer function circuit, and a supply of power to the dialer memory is taken effect from a memory back-up power source.
2. Description of the Related Art
Heretofore, in IC telephones in which various component parts of the individual circuit are integrated on a single chip, a dialer memory is mounted in a dialer which issues a dialer output signal upon receipt of a key input from a keyboard. In this dialer memory, various dialer signals such as a program, read-only data, etc. are stored. Accordingly a memory back-up battery is built in a circuit board to back up the memory in an effort to prevent the data from disappearing.
FIG. 6 of the accompanying drawings shows the general structure of a typical conventional dialer circuit in an IC form. As shown in FIG. 6, a keyboard 2 as an external circuit is connected to a dialer 4 where, in the tone output mode, a DTMF (Dual Tone Multi-Frequency) signal is outputted based on a key input signal K.sub.i given by depressing one of keys on the keyboard 2, and in the pulse output mode, a pulse signal corresponding to the key input is outputted to a telephone circuit line. In order to send out such a dialer output signal, the dialer 4 is equipped with a redial memory 401 and a repertory memory 402, as a dialer memory for storing dial data. Also in the dialer 4, a key input circuit 412, a system control circuit 414, an oscillator 416, a system clock generator circuit 418, an output control circuit 420, a DTMF output circuit 422, and a pulse output circuit 424 are integrated on one and the same semiconductor chip.
To the redial memory 401, repertory memory 402 and other dialer function circuits in the dialer 4, a supply voltage V.sub.L supplied via the telephone circuit line is supplied, as V.sub.DD, via a resistor 6, diode 7 and a capacitor 8. Further, in order to prevent the data in the redial memory 401 and the repertory memory 402 from disappearing, the dialer 4 is equipped with a battery 10 as a memory back-up power source. This battery 10 also is connected to the redial memory 401, the repertory memory 402 and other function circuits via a resistor 12 and a diode 13 in parallel to the line voltage V.sub.L.
However, in this conventional dialer circuit, as shown in FIG. 6, since the battery 10 is directly connected to the supply line of the supply voltage V.sub.DD given by the telephone circuit line voltage V.sub.L, an operating current flowing to the dialer circuit would be supplied from the battery 10 when the circuit voltage V.sub.L becomes lower than a voltage V.sub.M of the battery 10 (i.e., V.sub.M &gt;V.sub.L). Thus the battery 10, which is incorporated originally for the purpose of backing up the memory, would play here as a compensation power source to meet with the lowering of the circuit voltage V.sub.L. Consequently as the circuit voltage V.sub.L is blocked, the battery 10 would take a much amount of waste, thus resulting in a shortened span of life.